Integrated circuit chip manufacturers fabricate semiconductor devices by different combinations of processes. One widely used processing technique is chemical-vapor deposition (CVD) which is employed to form various material layers (metals, dielectrics, semiconductors) on the surface of semiconductor wafers. The material layers which manufacturers apply and etch may comprise various dielectric or insulating layers in addition to one or more of the following conductive layers: a thin metal coating such as tungsten, aluminum, copper, or gold; a thin polysilicon coating doped with conductive impurities; or other layers of metal silicides and metal nitrides. Process control and manufacturing tolerances apply to the sequential device fabrication processes. Deviations from specified target tolerances in excess of only a few percentage points during various device fabrication processes may result in defective and rejected semiconductor chips.
In thermal processing equipment such as in a single-wafer rapid thermal processing (RTP) reactor, one of the critical process parameters is the wafer temperature. Therefore, it is important to measure the wafer temperature and its distribution uniformity in real-time by a non-invasive temperature sensing device. Repeatable, precise, and process-independent measurements of the wafer temperature are among the most important requirements of semiconductor processing equipment (such as RTP) in integrated circuit manufacturing.
FIG. 1 shows an RTP reactor 20 containing a semiconductor wafer 22 and in which a typical non-contact temperature measuring pyrometer 24 detects radiance or black-body radiation emitted from the heated wafer. RTP reactor 20 has a process environment bounded by quartz (or metallic) process chamber (consisting of transparent walls 32, 36) contained within casing with a lower wall 28 and upper wall 40. Within RTP reactor 20 wafer 22 is heated on both sides by two banks of linear heating lamps (tungsten-halogen lamps) 30 and 38 through optical windows 32 and 36. The RTP system may employ only one bank of tungsten-halogen lamps for one-sided wafer heating. Thermocouple 34 may be used to provide wafer temperature measurement for calibration of the pyrometry readings. Since thermocouple requires wafer contact, it is not used during actual device processing.
Optical pyrometry has been used as a non-invasive method for wafer temperature measurement in the known RTP systems. However, the accuracy and reproducibility of conventional pyrometry are very sensitive to the wafer bulk and surface optical properties (or emissivity), interference due to the heating lamps, process environment, and the type of process being performed in the reactor. With the double-sided lamp heating arrangement, the pyrometer will usually experience direct radiation exposure from the lamps regardless of the positioning of the pyrometer. However, the disturbance of the pyrometer reading will be minimal if the spectral distribution of the heating lamps has no overlap with the pyrometer's operating spectral band or wavelength.
With the single-sided lamp heating (example shown with a single arc lamp) arrangement of FIG. 2 shown with a metallic process chamber, a hole 53 can be formed through a side of the RTP vacuum chamber 50 opposite the heat lamp 66 in order to insert an optical window (the single-sided RTP system 50 may use one bank of tungsten-halogen lamps instead of a single high-power arc lamp). Pyrometer 52 is placed near the hole 53 to detect a portion of the emitted black-body radiation 54 from wafer 22. This arrangement may be somewhat more suitable than the above-mentioned double heat lamp arrangement since it is free (not completely) from direct viewing of the lamp and its interference effects. However, silicon wafer 22 may remain at least partially transparent to the lamp radiation in the infrared region (e.g., beyond 1.5 .mu.m) at lower wafer temperatures (e.g., below 600.degree. C.), so pyrometer 52 may still be affected by lamp radiation passed through a partially transparent wafer 22.
Conventional pyrometry techniques also assume a semiconductor wafer has a known fixed emissivity. In actuality, emissivity can change from semiconductor wafer to semiconductor wafer. Emissivity values depend on various layers of materials present on the semiconductor wafer, substrate background doping, semiconductor wafer backside surface roughness, and semiconductor wafer temperature. The presence of device patterns on the semiconductor wafer and the pyrometry wavelength may also affect spectral emissivity. In practice, while the amount of error that using an erroneous value for wafer emissivity can produce is usually uncertain, it has been shown to cause wafer temperature measurement errors up to 10's or even 100's of .degree.C.
FIG. 3 is a graph of silicon wafer emissivity as a function of wavelength. For different wafer temperatures [in .degree.K.], FIG. 3 shows measured emissivity for a 1.77 mm thick silicon wafer with a relatively low level of n-type doping. The substrate is a comparatively pure sample having a resistivity of 1.4 .OMEGA.-cm. All measurements in FIG. 3 were made at a vacuum of 10.sup.-4 mm Hg. The plot of FIG. 3 shows measurements of spectral emissivity for silicon from 0.4 to 15.0 .mu.m at temperatures ranging from 543.degree. K. to 1070.degree. K. Thermal radiation of silicon is due to band-to-band transitions, free carriers and lattice vibrations. This radiation lies primarily in the visible and infrared regions of the spectrum. FIG. 3 shows that spectral emissivity changes significantly for pyrometry applications as a function of wavelength. As temperature changes, emissivity will change. Thus, it is not possible to accurately measure temperature of the semiconductor wafer using pyrometry, unless the pyrometry-based techniques compensate for changes in emissivity as a function of changes in temperature or surface optical conditions.
FIG. 4 illustrates the calculated relationship between the backside spectral 5.4 .mu.m emissivity and polysilicon layer thickness for a semiconductor substrate with two backside films and a bare front side. For a 500 .mu.m thick silicon substrate with a resistivity of 5 .OMEGA.-cm at 900.degree. K. with two material layers on the backside, FIG. 4 shows the spectral backside emissivity vs. polysilicon film thickness for various oxide layer thicknesses. For example, with a 100 .ANG. silicon dioxide first layer, the solid plot shows emissivity to be approximately 0.7 uniformly as polysilicon thickness increases from 0 to 1000 nm. With increased backside silicon dioxide layer thickness, emissivity will change significantly as a function of polysilicon layer thickness. In the extreme case shown, with a 5,000 .ANG. backside silicon dioxide, FIG. 4 (graph 72) shows that emissivity can range from 1.0 to approximately 0.25 with widely varying values therebetween. These emissivity variations can cause significant temperature measurement errors in conventional pyrometry.
Thus, there is a need for an improved and reliable method and apparatus to precisely measure the temperature of a wafer in a semiconductor device fabrication reactor.
Thus, there is a need for a method and apparatus to provide real-time in-situ non-invasive temperature measurements of semiconductor wafers during device fabrication processes.